Near-field communications, known by those skilled in the art under the acronym NFC, is a technology for wireless connectivity which allows a communication over a short distance, for example 10 cm, between electronic devices, such as for example contactless smartcards or mobile telephone emulated in card mode, and readers.
NFC technology is particularly adapted for connecting any type of user device and allows quick and easy communications.
A contactless object is an object capable of exchanging information via an antenna with another contactless object, for example a reader, according to a contactless communications protocol.
An object which is a contactless object is an object compatible with NFC technology.
NFC technology is an open technology platform standardized in the ISO/IEC 18092 and ISO/IEC 21481 standards but incorporates numerous already existing standards such as for example the type A and type B protocols defined in the ISO-14443 standard which are communications protocols that may be used in NFC technology.
Aside from its conventional telephone function, a cellular mobile telephone may be used (if it is equipped with specific means) for exchanging information with another contactless device, for example a contactless reader, based on a contactless communications protocol usable in NFC technology.
This allows information to be exchanged between the contactless reader and secure elements situated within the mobile telephone. Numerous applications are thus possible such as mobile ticketing in public transport (the mobile telephone behaves as a travel ticket) or else mobile payment (the mobile telephone behaves as a payment card).
When information is transmitted between a reader and an object emulated in tag or card mode, the reader generates a magnetic field by means of its antenna which is generally, in the standards conventionally used, a sine wave (the carrier) at 13.56 MHz.
In order to transmit information from the reader to the object, the reader uses an amplitude modulation of the carrier.
For its part, the object demodulates the received carrier so as to deduce from it the data transmitted from the reader.
For transmission of information from the object to the reader, the reader generates the magnetic field (the carrier) without modulation. The antenna of the object emulating the tag then modulates the field generated by the reader, as a function of the information to be transmitted. The frequency of this modulation corresponds to a sub-carrier of the carrier. The frequency of this sub-carrier depends on the communications protocol used and may for example be equal to 848 kHz.
This modulation is carried out by modifying the load connected to the terminals of the antenna of the object.
By modifying the load across the terminals of the antenna of the object, the output impedance of the antenna of the reader changes owing to the magnetic coupling between the two antennas. This results in a change in the amplitudes and/or the phases of the voltages and currents present on the antennas of the reader and of the object. Accordingly, in this way, the information to be transmitted from the object to the reader is transmitted by load modulation to the antenna current of the reader.
The variation in load carried out during the load modulation results in an amplitude and/or of phase modulation of the signal (voltage or current) on the antenna of the reader. A copy of the antenna current is generated and injected into the receiver chain of the reader where it is demodulated and processed so as to extract the transmitted information.
The best transfer of power between the reader and the tag is obtained when the tag is equipped with a circuit matched with the resonant circuit of the reader, and itself resonant at the frequency of the signal transmitted by the reader, for example 13.56 MHz.
Before the reader and the object communicate according to a contactless communications protocol, a phase is provided for detection by the reader of the potential presence of the object close to the latter.
Such a mode is for example described in the contactless communication standards and includes an emission by the reader, more or less often, for example every second, of requests and wait periods for specific responses coming from the object.
If these specific responses are received, then the object is detected and the communication can begin.
However, such a detection mode consumes a lot of power, which is detrimental for a reader operating on battery power.
There currently exists a low-consumption mode for detecting objects which consists, in a calibration phase, in emitting pulses of magnetic field at a frequency of 13.56 MHz and in measuring the amplitude of the electromagnetic field on the antenna. Indeed, if a card comes close to the antenna of the reader, this results in a voltage drop on the antenna of the reader due to a coupling effect.
The calibration phase therefore supplies a reference value for the amplitude of the magnetic field measured on the antenna.
Then, aside from calibration, if at a given moment in time the measured amplitude of the electromagnetic field on the antenna of the reader changes from this reference value, then this means that there has been a radiofrequency phenomenon (coupling effect) which may be due to the potential presence of a contactless object or else to the presence, for example, of a mass of iron in the environment of the reader.
A verification is then carried out for the effective presence of the object using the standardized procedure, in other words the sending of requests and waiting to receive acknowledgement signals.
If this verification phase arrives at the conclusion that the coupling effect obtained during the detection phase did not correspond to an effective detection of an object, a calibration phase is then repeated in order to obtain a new reference value.
Such a detection has drawbacks.
One reason is that the reader is equipped with a receiver chain comprising a demodulator which is set to the frequency of the sub-carrier used by the object for transmitting information to the reader.
Thus, this sub-carrier is for example equal to 848 KHz in the type A and type B protocols defined in the ISO-14443 standard.
However, if the measurement of the amplitude of the electromagnetic field on the antenna is carried out by specific means without using the receiver chain, then the sensitivity of the detection is well below the sensitivity of the receiver chain during the reception of the data from the object to the reader.
In other words, although the object could perfectly well communicate with the reader at a certain distance, the detection of the presence of the object would only be effective at a shorter distance.
If, on the other hand, the measurement of the amplitude of the electromagnetic field is carried out using the receiver chain, the detection signal will then be extremely weak owing to the filtering of the 13.56 MHz carrier frequency by the receiver chain.
This will therefore result in difficulties in the detection of the object.